Page:Popular Science Monthly Volume 16.djvu/557

 blacks of Soodan and the various peoples of white race driven into the desert by successive invaders. Diodorus, a priest of Tarsus in the fourth century, speaking of the great oasis in the desert forty leagues from the Egyptian frontier, mentions it being irrigated, not by rivers nor by rains, but by springs that issue from the ground not spontaneously, nor in consequence of the rains sinking into the ground, but by great labor on the part of the inhabitants. Several wells alluded to by him have been cleared since 1849 by a French chemist, M, Ayme, who established alum-factories in two Egyptian oases. These old wells were fitted with a stone pear-shaped valve by which the issue could be regulated.

About the middle of the sixth century, Olympiodorus of Alexandria speaks of wells five hundred cubits deep. Arabian writers in the middle ages describe them in detail; their great historian, Ibn Khaldoun, speaking of the spouting wells of the Sahara, considers them "a miraculous fact."

The origin of these subterranean waters is now well known. The streams flowing down the southern slopes of the Atlas Mountains of Morocco, Algeria, and Tunis, and on all sides of the Tibesti, Hogar, and other Saharan mountains, quickly disappear through the sands. M. Vivien de Saint-Martin, in "Le Nord de l'Afrique dans l'antiquité," says that, "under the sandy crust through which the waters necessarily sink, layers of clay have been found everywhere at various depths underground, where sheets of water make actual rivers."

The natural question then arises as to what causes these streams; how the parched desert furnishes rivers? Rains are quite abundant on the summits of the mountains—so much so that, in winter especially, the streams attain considerable proportions. The Sahara experiences at times tremendous storms and torrents of rain that in a few moments cause violent freshets. Dr. Barth, in his "Reisen in Nord und Central Afrika," cites among others a deluge that he witnessed at Tintagoda in latitude 19°. In less than an hour after a heavy rainfall on the mountain a sheet of water was rushing by with such force as to carry away herds of cattle and uproot trees; it covered to a considerable depth the whole valley, over a mile broad. In "Les Touaregs du Nord," M. Duveyrier says: "I had occasion on the 30th of January, 1861, while at Oursel, at the foot of the Tasli Mountains, to observe the overflowing of one of the numerous torrents that descend from that mountain. The rapidity of the stream was a metre a second, and the water brought down such alluvia that afterward the Touaregs could sow cereals where before there had been no arable ground." Further on the same traveler says: "In the spring of 1862 a storm of rain falling on the southern slopes of the Ahaggar brought such quantities of water into the valleys of Idjeloudjal and Tarhit that a portion of the mountain was carried away. The action of the water was so rapid as to sweep away and destroy an entire tribe encamped at the